isolated power for digital meter

[James Sweet]

I have a small LCD panel meter which I'd like to use in an application in
which the power supply and the voltage being measured share the same ground.
I know I've seen circuits before to generate an isolated voltage of 9V or so
at a few mA for this application but can't find one at the moment.

 

[James Sweet]

http://tinyurl.com/2pwntw

That's not an acceptable solution, this needs to be externally powered, no
battery to replace.

 

[Bill Kaszeta / Photovoltaic Resources]

If your DC voltage is at least that required to operate the LCD meter and you
can accept a pust-to-test 'feature', then you can use a double-pole/2-position
push button switch, resistor, a zener diode, and a capacitor.

The capacitor goes across the common poles of the switch (watch polarity),
the normally closed poles of the switch go to a charging circuit, and the normally
open poles of the switch go to the LCD meter power inputs. If the voltage to be
measured is higher than the allowed limit for the LCD power, use a series
resistor and a zener diode across the capacitor to limit the voltage.

The capacitor is charged when the push button switch is not pushed. When the
push button switch is pushed, the capacitor is briefly isolated, then connected to
the LCD meter. Years ago I used with a 250 MF 16V capacitor and each push
would operate the meter for 10-15 seconds. Meters these days may require
less power.

I have a small LCD panel meter which I'd like to use in an application in
which the power supply and the voltage being measured share the same ground.
I know I've seen circuits before to generate an isolated voltage of 9V or so
at a few mA for this application but can't find one at the moment.

Bill Kaszeta
Photovoltaic Resources Int'l
Tempe Arizona USA
[email protected]

 

[James Sweet]

Something like this? Cheap enough for you not to have to get PCBs made,
transformers wound & EMC test.
http://uk.farnell.com/jsp/Electrica...GIES/NMA1209SC/displayProduct.jsp?sku=1021437
Or perhaps some photovoltaic cells lit by LEDS?

That link doesn't seem to be working.

At any rate I think you guys are making this out to be more complex than it
really is. I saw a few circuits at one time that used a little home made
transformer, a couple of transistors, diodes and a capacitor to form a
little self oscillating inverter. I think I saw another one that used a 555
and a few capacitors. It's the sort of thing I can hack together on a scrap
of perfboard using junkbox parts. I just figured I'd ask around for a tested
schematic rather than reinvent the wheel and design one myself. I only need
one, this isn't something I'd have a pcb made.

 

[ehsjr]

That link doesn't seem to be working.

At any rate I think you guys are making this out to be more complex than it
really is. I saw a few circuits at one time that used a little home made
transformer, a couple of transistors, diodes and a capacitor to form a
little self oscillating inverter. I think I saw another one that used a 555
and a few capacitors. It's the sort of thing I can hack together on a scrap
of perfboard using junkbox parts. I just figured I'd ask around for a tested
schematic rather than reinvent the wheel and design one myself. I only need
one, this isn't something I'd have a pcb made.

A few years back I made one of those with a 2 transistor
multivibrator with a small center tapped 600 ohm modem
transformer as the load. A zener, diode resistor and cap
on the output side provided the 9V.

+---->|-----+----[R]---+--- +9
| | |
| [22uF] [Zd]9v
| | |
+-uuuuuuuuu-+----------+--- 0V
===========
+---+---------uuuu-+-uuuu-------+---+
| | | | |
| +--------------------+--||--+ |
| | | .1uF | |
| | | | |
| +----------- | -----+ |
c\ | | | /c
|--[10K]--+ [60R] +--[10K--|
e/ | \e
| o + Vin |
| |
+------------------+----------------+
|
Gnd

Vin was tested from 6 to 16 volts and the circuit maintained
9 volts out at 10 mA. The transformer provided the R to limit
the current through the zener.

Ed

 

[James Sweet]

Do you have any old ethernet cards, with the 5 VDC to isolated 9 VDC
coverter modules?

Ooh, I do! I hadn't even thought of that, I've got a whole pile of them.